1. Field of the Invention
This invention generally relates to analog filters, and more particularly to tuning an analog filter using a digital circuit.
2. Description of the Related Art
Continuous-time, or analog, filters are often used in wireless devices such as cellular telephones. Typically, the analog filter and other circuits comprising a receiver or a transmitter are on a single integrated circuit manufactured using complementary metal oxide semiconductor (CMOS) technology. The analog filter comprises components, including resistors and capacitors, that affect a bandwidth of the analog filter. Integrated circuit manufacturing or fabrication processes may cause the actual value of such components to vary by as much as 30% from their nominal value, which may cause bandwidth variations by as much as 50%. The bandwidth variations may also be caused by temperature or voltage changes. Variations in the bandwidth of the analog filter may lead to significant performance degradation in both receive and transmit signal paths of the wireless device. In the receive signal path, variations in the bandwidth of a baseband analog filter leads to performance degradation in static sensitivity, sensitivity in the presence of interferers, receiver third-order intercept point, anti-aliasing performance and error vector magnitude (EVM). In the transmit signal path, variations in the bandwidth of the baseband analog filter leads to performance degradation in the EVM, adjacent channel leakage ratio, and static/transient power mask performance.
Bandwidth tracking accuracy and performance are essential in wideband code-division multiple access (WCDMA or 3G) and high-speed downlink packet access (HSDPA or 3.5G) receivers to preserve necessary 0.1% bit error rate (BER), 5% EVM, interferer rejection, and analog-to-digital (A/D) anti-aliasing protection. In known 3G and 3.5G transceivers, the 0.1% BER sensitivity is degraded by as much as 0.5-decibel (dB) and EVM is degraded from 2% to 8.4%, due to large receiver bandwidth tracking errors of up to 12.5%.
A tracking loop is generally used to vary R/C filter parameters of an analog filter. The tracking loop tracks the variation in component values that may occur. The tracking accuracy of some known methods is limited due to a variance in a pseudorandom calibration signal and due to long term averaging needed for the tracking loop to converge. The total calibration time of known methods that use a pseudorandom calibration signal is approximately 10-msec, which is disadvantageously long. In spite of the long calibration time of known methods, it is difficult for known methods to achieve high calibration accuracy because of a variance in the pseudorandom signal.
Other known bandwidth tracking techniques focus on the concept of master-slave tracking. Such techniques use a filter stage configured as an oscillator with the exact same topology as the circuit used in sections of a main filter. Any manufacturing process and/or temperature variations should affect the main filter and the slave circuit by the same amount. This technique establishes, in essence, a phase-locked loop around the slave and keeps the oscillation of the oscillator (or the phase difference of the filter) always close to a stable value by tuning all the resistors (or capacitors) of the main filter. Such techniques rely on a matching between the various sections of the main filter and the slave circuit. However, precise matching is not always possible because the main filter occupies a different portion of a die than does the oscillator, and the lack of matching leads to performance degradation in the tracking accuracy.
Other known designs use an in-band tone and a band-edge tone to tune a filter. In such designs, the in-band tone provides a reference against which the band-edge tone is measured. Such designs require additional time because separate, non-concurrent measurements are required because the signals are not presented in a composite format. Furthermore, correction accuracy is lost because the slope of the magnitude response of the filter is low. In addition, as the filter is tuned lower in frequency, the amplitude of the band-edge calibration signal drops off, further reducing the resolution.
Some known resistor/capacitor (R/C) tuning systems use a dedicated analog oscillator that tracks the R/C time constant stages (biquad and mixer pole) that require tuning. The R/C time constant of the oscillator is measured using a comparator output that forces control logic of the tuning system to check the value of a digital timer and determine whether the R/C time constant of the oscillator is tuned optimally. Based on whether the R/C time constant is too slow, too fast, or within tolerance, a digital accumulator is decremented, incremented, or left unchanged, respectively. Disadvantageously, dedicated analog circuitry is required to perform the R/C time constant measurement, and the complexity of the required analog circuitry is more critical than digital circuitry. An increase in analog circuitry disadvantageously increases design time, die area and current drain.
Known digital tracking methods use a fast Fourier transform (FFT) method for power detection; however, the FFT method disadvantageously causes a significant amount of hardware cost and current drain.
Furthermore, all known methods lack any dynamic control of the quality factor (Q) of the active filter, to improve tracking performance.